The present invention relates to composite friction elements for brakes and clutches and pertains particularly to improved friction elements, composition and method for making same.
A friction brake is basically a pair of friction members, one rotating and one stationary, brought into engagement to produce a friction force measured as brake torque for either slowing or stopping the rotating element. Brakes are preferably designed so that the brake torque is somewhat proportional to the input force used to engage the elements and the energy of the rotating member is dissipated in the form of heat. Unfortunately, pressure is not the only factor that influences the frictional response of the brake elements. Friction effects between friction elements cause friction force and brake torque to vary with engaging pressure, speed, and temperature, and to depend upon deposited interfacial film for stability. Nevertheless, brakes are preferably designed so that the brake torque is reasonably proportional to the input force used to engage the elements. The energy of braking is dissipated in the form of heat through the brake elements. For this reason they must be able to withstand a great deal of heat for most applications.
The rotating element of a brake system is usually a disc or drum made of metal such as a steel, and the stationary element is usually a composition pad or shoe lining moveable into and out of engagement with the rotating element. The composition element is designed to wear without undue wear of the metal disc or drum. The materials forming the composition element are the principle unpredictable variables that have the greatest affect on the performance characteristics of the brake system. Desirable materials for the composition element must be safe to use, relatively inexpensive, have good friction, wear and heat performance characteristics. This includes good fade resistance, or the ability to maintain good (preferably substantially uniform) braking with heat buildup.
A friction clutch is similar in some respects to a brake and is basically a pair of friction elements designed to selectively couple a rotating driving element to a driven element to bring the driven element up to speed to rotate with the driving element. The clutch elements, both rotating with one driving and one driven, are brought into engagement to produce a friction force driving torque for bringing either a slow moving or a stationary element up to speed with a rotating driving element. The clutch usually consists of a circular friction plate or disc having friction pads or elements of composition material on both faces squeezed between a pair of metal pressure plates.
Until recent years, the predominant reinforcing material used in the manufacture of friction pads and discs for brakes, clutches and the like was asbestos. These pads were manufactured by a molding process where each unit was formed of a composition of randomly oriented asbestos fibers in a bonding matrix placed under pressure in a mold cavity. However, it was discovered that asbestos is a carcinogenic substance, and that such use released potentially harmful amounts of it into the environment. For this reason, some industrialized countries prohibit the use of asbestos friction materials, and others including the United States require the use of asbestos to be phased out over the next few years. Therefore, there exists an urgent need for safe and effective friction materials and economical methods of manufacturing the materials into suitable friction units.
Extensive efforts have been put forth in recent years in an effort to find suitable environmentally safe materials and compositions having the desirable wear, heat and other characteristics to serve as a substitute for asbestos as friction elements in brakes and clutches. These efforts have been frustrated by the many and varied parameters involved, including the range of needs to be met between brakes and clutches as well as different types of brakes and different types of clutches. For example, different size vehicles require different size friction pads for both brakes and clutches and often have other variables including higher operating forces and temperatures. Brake pads used with rotating discs have different conditions than shoes used with brake drums. Also clutches used with automatic transmissions have different conditions from clutches used with manual or stick shift transmissions.
Attempts to satisfy the need for long life, high friction heat resistant friction materials have included proposals to utilize various chopped fibers molded in a bonding matrix, such as a resin. The friction unit is formed in the traditional fashion by a molding process, with the fibers and other components randomly oriented and placed in a binder, such as either a dry powder resin cured under heat and pressure, or placed in a liquid resin in a mold and cured. Examples of these compositions and manufacturing methods are disclosed in U.S. Pat. No. 4,119,591, granted Oct. 10, 1978 to Aldrich, U.S. Pat. No. 4,259,397, granted Mar. 31, 1981 to Saito et al., and U.S. Pat. No. 4,432,922, granted Feb. 21, 1984 to Kaufman et al.
However, friction units made by this method are expensive to manufacture and have not been satisfactory, because of their lack of uniformity in performance and durability. For example, units from the same batch may vary as much as 35% in performance characteristics. The non-uniformity of results has been found to be caused largely by a non-uniformity of distribution and orientation of the fiber and other components in the matrix. This not only creates expensive inspection and quality control problems, it can also create maintenance problems, and sometimes even hazardous conditions. For example, pads that have been matched for performance at initial installation may vary over their useful life.
Throughout the past several years, the applicant has developed extensive improvements in compositions and structures as well as in pultrusion methods of manufacture of composite friction elements for brakes and clutches. Many of these improvements are embodied in the applicants U.S. Pat. Nos., 5,156,787, entitled “PULTRUSION METHOD OF MAKING BRAKE LININGS”; 5,462,620, entitled “CONTINUOUS PULTRUSION METHOD OF MAKING FRICTION UNITS”; 5,495,922, entitled “UNIFORM COMPOSITE FRICTION UNITS; and 5,690,770 entitled “PUL TRUSION METHOD OF MAKING COMPOSITE FRICTION UNITS. However, the applicant's continuous work on perfecting these compositions, structures and methods indicate that further improvements in both compositions and methods of manufacture are desirable and consequently have been developed by the applicant. For example, improved mechanical properties, compositions and structures were developed as well as improvements in pultrusion manufacturing methods.
Now then, further improvements are desirable in compositions, structures and methods of manufacture. Accordingly, it is desirable that improved compositions, structures and methods of manufacture be available to overcome the above and other problems of the prior art.